1. Field of the Invention
The present invention relates to purification and filtration of molten polymer and, more particularly, to an improved method and apparatus for providing variable filtering capacity in a continuous polymer extrusion process.
2. Description of the Related Art
In processes involving extrusion of molten thermoplastic polymers such as polyethylene, Nylon, polyester, polystyrene, etc., it is necessary to filter foreign matter (i.e., contaminants) from the molten polymer. A variety of polymer filtration systems is known in the prior art for removing impurities from molten polymer. Two common types of polymer filters are screen changer systems and candle filter systems. Screen changers use screens to remove dust, dirt and foreign particles, and typically have a relatively small area of screen for a given flow rate of molten polymer. A candle filter system typically has two or more filter housings and uses valves to direct polymer to and from the filter housings. Each housing contains multiple candle-type filter elements. The candle filter element is a perforated tube covered by pleated screen wire in two or more layers. The candle filter system is normally used for high polymer flow rate and/or very fine filtration systems.
A very popular brand of the candle-type filter system is the Fluid Dynamics CPF system, which is manufactured by USF Filtration & Separation, Inc. It has two filter housings and uses two sliding spool valves to direct the polymer flow to and from the filter housings. During normal operation, one of the two filter housings is on-stream (i.e., molten polymer is flowing therethrough); the other filter housing is cleaned, installed and heated to be ready to accept the polymer. When the on-stream filter becomes too dirty for continued operation, spool valves of the system are set in motion in the following sequence: (1) the inlet valve of the clean filter housing is slightly opened while the outlet valve of the clean filter housing remains closed to allow the polymer fluid to enter and fill the clean housing; (2) the trapped air in the clean housing is purged through a bleed port until all air is vented from the clean housing; (3) after the clean housing is completely filled with molten polymer, the bleed port is closed and then the outlet valve of the clean housing is fully opened; and (4) the inlet valve of the clean housing is fully opened, after which the inlet and outlet valves of the dirty housing are completely closed. This completes the switching of the polymer fluid from the dirty housing to the clean housing, and the filter of the dirty housing can then be removed for cleaning or replacement. While the clean housing is being filled, the filter element in the dirty housing continues to provide uninterrupted process filtration.
U.S. Pat. No. 5,462,653 to Hills, the disclosure of which is incorporated herein by reference in its entirety, discloses a large area polymer filtration system having a generally cylindrical shape with six candle-type filter housings arranged in pairs in a ring about a central valve and distribution system. A rotatable control plate controls the valve and distribution system and can be set in various positions to allow polymer flow through all of the filter housings or to prevent flow through individual pairs of filter housings while the other housings remain on-stream, in order to permit removal or replacement of clogged or dirty filters. While the control plate can be positioned to prevent flow to individual pairs of filter housings, the filter housings are not individually controllable to be on-stream or off-stream independent of other filter housings (i.e., either four or six filter housings must be on-stream, and filters can be taken off-stream only in pairs, one pair at a time).
Normally, the size of a polymer filtration system is chosen to provide sufficient filtration for the polymer process system at its maximum flow rate. Under certain operating conditions or for certain processes, the process system may be required to run at a reduced capacity, for example, in a process system having multiple functions or in systems producing plural-component polymer products. One problem resulting from running the process system at a reduced capacity or variable capacity is that the molten polymer remains within the filtration system for a relatively long period of time (i.e., the polymer has a high polymer "residence time"). Long residence time and non-uniform residence time can potentially cause thermal degradation of the polymer when thermally sensitive polymers are used.